It is the fdm that is causing the transfer of fuel. The fdm is the part the pump is in and controls which tank gets the return fuel. It's called the fuel delivery module.
Why are you removing the side tank. Is it bad or what. It makes more sense to replace it and have the additional fuel available. And they aren't much money. The fdm is where the investment comes.
But if you must remove it, then cap the hoses off real well, and that's all you need to do.

In place of the side tank I will be able to install a larger on board air tank. That will come in handy for farm work.

I'm also in the process of rewiring the truck so only having one in tank fuel pump to wire will make things a little more simple.

My air tank is inside the quarter panel out of the way.
As far as your rewiring goes, what's wrong with current wiring that you're going to be redoing fuel pump wiring. The switch in the dash does fuel level and pump.

That's a good idea to hide the air tank in the quarter panel. However, my truck has a flatbed and I have underbody tool boxes so I'm running out of room to hide the air tank.

I'm completely restoring this truck so the wiring has to go. The previous owner cut, rewired, used household wire, etc. to make repairs. That has caused lots of electrical gremlins that make the truck unreliable and frustrating. Rewiring to a single large tank in the rear just makes sense for this application.

That's a good idea to hide the air tank in the quarter panel. However, my truck has a flatbed and I have underbody tool boxes so I'm running out of room to hide the air tank.

I'm completely restoring this truck so the wiring has to go. The previous owner cut, rewired, used household wire, etc. to make repairs. That has caused lots of electrical gremlins that make the truck unreliable and frustrating. Rewiring to a single large tank in the rear just makes sense for this application.

Out of curiosity, is your truck a cab and chassis, or a pickup that's had a flatbed installed? Reason I ask, is that the frame rails on a cab and chassis truck are spaced differently than a pickup. Cab and chassis are narrower.

Out of curiosity, is your truck a cab and chassis, or a pickup that's had a flatbed installed? Reason I ask, is that the frame rails on a cab and chassis truck are spaced differently than a pickup. Cab and chassis are narrower.

Has anyone done this mod on a diesel?
I have a '94 F-350 CC Dually with the dual tanks. My rear tank is the 19 gal. I think.
I'd love to upgrade to a larger tank, and since the diesel does not use an in tank pump, I'm thinking that extending the length of the pickup tube and slight modification to the gauges float rod assembly would be all that's required to make it work.
Just wondering if anyone else had done it and if so, what had to be changed (besides the tank & lower straps).

Has anyone done this mod on a diesel?
I have a '94 F-350 CC Dually with the dual tanks. My rear tank is the 19 gal. I think.
I'd love to upgrade to a larger tank, and since the diesel does not use an in tank pump, I'm thinking that extending the length of the pickup tube and slight modification to the gauges float rod assembly would be all that's required to make it work.
Just wondering if anyone else had done it and if so, what had to be changed (besides the tank & lower straps).

I've done this on a couple of trucks. Both 38 gallon tanks I bought came with instructions and hardware to do the swap, so you won't need to buy anything else. I had to extend the float so it had full range of the tank, you'll probably have to play with the adjustment so the fuel gauge will read correctly, other than that it is a straight forward swap.
I think Rockauto has the best pricing on the tank.

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